1. Field of the Invention
The present invention relates to an image-taking apparatus, and more particularly to an image-taking apparatus that takes in an image of a subject optically through a zoom lens system and then outputs it in the form of an electrical signal by means of an image sensor, and among others to an image-taking apparatus provided with a compact, lightweight zoom lens system.
2. Description of the Prior Art
Today, digital cameras are quite popular. Digital cameras dispense with silver-halide film, and use instead an image sensor such as a CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) sensor to convert an optical image into an electrical signal so that the optical image can be recorded and transferred in the form of digital data. A digital camera incorporates an image-taking apparatus provided with a lens system and an image sensor, and, in recent years, such image sensors have come to have increasingly large numbers of pixels. Correspondingly, the demand for high-performance image-taking apparatuses has been increasing greatly. Among others, compact image-taking apparatuses have been eagerly sought after that incorporate a zoom lens system that permits zooming without degrading image quality. In addition, in recent years, as the image processing performance of semiconductor devices and the like increases, more and more personal computers, mobile computers, cellular phones, personal digital assistants (PDAs), and the like have come to be internally or externally fitted with an image-taking apparatus having a zooming capability. This has been spurring on the demand for compact, high-performance image-taking apparatuses.
To make an image-taking apparatus compact, the zoom lens system incorporated therein needs to be made compact. Thus, many zoom lens systems have been proposed that aim at compactness. For example, zoom constructions including, from the object side, at least a first lens unit having a negative optical power and a second lens unit having a positive optical power (so-called “negative-led” zoom lens systems) are proposed in the following patent publications:                Publication 1: U.S. Pat. No. 5,357,374        Publication 2: U.S. Pat. No. 5,745,301        Publication 3: U.S. Pat. No. 6,040,949        Publication 4: U.S. Pat. No. 6,229,655        Publication 5: U.S. Pat. No. 6,532,114        Publication 6: Japanese Patent Application Laid-Open No. 2002-72095        Publication 7: Japanese Patent Application Laid-Open No. 2003-177314        
Patent publications 1 to 7 all disclose zoom lens systems that employ a plastic lens element as the lens element disposed at the object-side end thereof. Using a plastic lens element makes it easy to introduce an aspherical surface, and helps reduce costs. However, with the constructions disclosed in patent publications 1 to 7, it is difficult to achieve compactness while maintaining high optical performance. For example, in the zoom lens systems disclosed in patent publications 1 and 2, the first lens unit is composed solely of a single lens element and is kept in a fixed position during zooming. This results in insufficient correction of aberrations. Also in the zoom lens systems disclosed in patent publications 3 and 7, the first lens unit is composed solely of a single lens element, resulting in insufficient correction of aberrations. In the zoom lens systems disclosed in patent publications 4 to 6, the first lens element is a plastic lens element. This lens element, however, is given too weak an optical power to achieve satisfactory compactness.